The present invention relates to the extraction of silica from rice bran, and more particularly, to a method for extracting high purity amorphous silica or solid silica from rice bran which is harmless to the environment, of which a by-product from rice bran can be utilized.
An invention directed to a method for extracting amorphous silica has been filed in China (Chinese Patent Application No. 86-104705), and granted to a patent in India under registration No 159017, and also issued as a Russian patent No. 2061656.
The purity of the silica produced by using the above method is about 82xcx9c99.9%, which purity is, however, not suitable for silica in demand in the semiconductor field.
As for the silica produced by the above methods, since the content of coke that remains after burning the rice bran is high, it makes it impossible to obtain a high purity silica.
In addition, when rice bran is burned, soot is discharged, and about 32% CO and CO2 are contained in the smoke from burning rice bran, which is harmful to the environment.
Meanwhile, Korean patent publication No. 97-5090 discloses a method for extracting silica without deforming the structure of silica formed during a growth period of rice by processing rice bran or rice straw with an enzyme and a method for adjusting the silica-carbon ratio by controlling the amount of enzyme and a reaction time.
However, the disclosed methods are directed to the matter of the silica structure, and a residual carbon amount is more than 14 wt %, very high, without description of any method for extracting a high purity silica as required in the present invention.
Moreover, Korean patent publication No. 97-5090 reveals that a Chinese Patent (CN 1,039,568) and a Russian Patent (SU 1,699,918) disclose a method for burning rice bran after processing in acid at a temperature of 700xcx9c900xc2x0 C. to destroy the cellulose crystallization, but without mentioning any method for fabricating a high purity silica from rice bran which are washed with an acid solution, dehydrated under pressure and then burned in a second step, as in the present invention.
Furthermore, Russian Laid-Open Publication No. 94031518 A1 of Jul. 10, 1996, German Laid-Open Publication No. 1532398A of Nov. 15, 1978, a French Laid Open Publication No. 2356595 A1 of Mar. 3, 1978 also disclose a method in which rice bran is sieved to remove impurities, washed with water, washed in an acid solution, dried, burned and pulverized. The silica product obtained by this method has a purity of 99.0xcx9c99.5%, but by-products of phytine, phytic acid (C6H6[OPO(OH)6] and xylitol (CH2OH(CHOH)3CH2OH) are generated from the soot.
Accordingly, a method for producing high purity silica, which is capable of easily obtaining silica of 99.99% purity, increasing the purity of a silica product by discharging soot generated as a by-product during a burning process of rice bran by using a fan, separating phytic acid and xylitol included in supernatant of a storage tank in which soot has been filtered to be removed, solving an environmental pollution problem due to the by-products by utilizing the soot settled at the bottom of the storage tank as an antiseptic solution, and obtaining a useful product, is highly demanded.
Therefore, an object of the present invention is to provide a method for extracting silica of 99.99% purity from rice bran, which is capable of removing soot, a by-product harmful to the environment, to increase the purity of the silicon product and producing a useful product from the soot.
In order to achieve the above objects, there is provided a method for extracting silica of more than 99.99% purity from rice bran including: the steps of: (a) pretreating rice bran by washing rice bran and removing impurities floated on the water; (b) sequentially washing the rice bran in an acid solution and water at least twice; (c) dehydrating the washed rice bran under pressure; (d) drying the dehydrated rice bran; (e) priomarily burning the dried rice bran at a temperature of 350xcx9c400xc2x0 C. while agitating and pulverizing; and (f) secondarily burning the rice bran at a temperature of 700xcx9c1000xc2x0 C. for 10xcx9c60 minutes while agitating by providing oxygen or air.
The present invention provides a method for extracting silica of 99.99% purity which is capable of effectively removing soot, a by-product, and obtaining a useful chemical product from such by-product.
In detail, the method for extracting silica of 99.99% purity includes a pre-treatment step (a) in which rice bran is sieved to remove impurities and washed in water to remove impurities floated on the water; a washing step (b) in which the rice bran is sequentially washed in an acid solution diluted with distilled water and then in distilled water at least twice; a step (c) in which the washed rice bran is dehydrated under pressure; a step (d) in which the dehydrated rice bran is dried; a first burning step (e) in which the dried rice bran is burned at a temperature of 350xcx9c400xc2x0 C. while being agitated and pulverized; a second burning step (f) in which the rice bran is burned at a temperature of 700xcx9c1000xc2x0 C. for 10xcx9c60 minutes while being agitated by providing oxygen or air.
The processes for extracting high purity silica from rice bran will now be described in more detail.
(a) Pre-Treatment Step:
A quantity of rice bran is sieved to filter out waste matter and then washed in water. At this time, impurities floated on the water are removed. During the washing process, most impurities are removed. These impurities being removed at this time are hardly removed even at a second burning step wherein the rice bran is treated at the high temperature of 700xcx9c1000xc2x0 C., remaining in silica. Thus, removal of the impurities in the washing process is very critical. During the washing process, if the impurities floated on the water are not removed and then the moisture is removed when extracting silica, the finally obtained silica would have a low purity.
During the washing process, the rice bran may be washed in cold water. But when washed in hot water at 85xcx9c95xc2x0 C., the impurities are more effectively removed. In the case of using hot water at 85xcx9c95xc2x0 C., the rice bran is washed by being continuously agitated. Use of hot water disadvantageously incurs a higher production cost. Thus, the combined use of cold water and hot water may be possible in consideration of economical efficiency and desired purity. The water used may be ordinary water, but distilled water is preferred in view of the purity of the obtained silica and the follow-up processes.
In one embodiment of the present invention, if the rice bran is washed in boiling water, that is, boiling water at a temperature of 100xc2x0 C., for 30 minutes to 120 minutes, since the bubbles generated when water is boiled agitate the rice bran, the rice bran can be advantageously washed without using an additional agitator. Accordingly, as the time period is reduced from 120 minutes to 30 minutes, the purity of the silica is heightened. In addition, in case of washing the rice bran in boiling water, the water obtained after the rice bran is washed includes much phytic acid and xylitol component while the washed rice bran have less organic substances. And soot is reduced, and in case of discharging gas, less poisonous gas (CO, NO2) is contained therein.
Before washing the rice bran in an acid solution, a process in which the structure of the rice bran is changed to facilitate removal of organic substances may be arbitrarily added. That is, if the washed rice bran is left at a temperature of xe2x88x9210xcx9cxe2x88x92140xc2x0 C. for 10xcx9c300 minutes, the final purity of the silica is heightened.
As the liquid contained in the rice bran is frozen, the structure of the rice bran is broken, changing to a form which is easily broken. Thus, after the freezing process, when the rice bran is washed in an acid solution, the organic substances contained in the rice bran can be more easily and quickly removed.
In order to freeze the rice bran, a freezing method known to the persons skilled in the art may be used.
Before washing the rice bran in an acid solution, when the rice bran is left at a low pressure, that is, for example, in a closed chamber in the range of 0.5xcx9c200 Pa, and preferably, in the range of 0.5xcx9c100 Pa, liquid contained in the rice bran is evaporated, causing an excessive pressure in the rice bran.
Thanks to the excessive pressure, the structure of the rice bran is broken, so that the organic substances in the rice bran can be more easily and quickly removed when washed in the acid solution.
If the pressure in the closed chamber is less than 0.5 Pa, the purity of the finally obtained product is not further heightened, while, if the pressure exceeds 200 Pa, vacuum effect does not affect the purity of the product any more.
(b) Process for Washing Rice Bran in Acid Solution:
The rice bran is washed in an acid solution to sterilize the rice bran and dissolve out the impurities. For example, as the acid solution, solution of sulfuric acid, nitric acid or acetic acid may be used, and it has no relation to the kinds of acids to be used.
The higher the acid concentration of the acid solution, the higher the purity of the finally obtained silica.
The rice bran is washed by being agitated in an acid solution of distilled water to remove impurities, and then washed again in the distilled water. In order to obtain a finally higher purity of silica, preferably, the washing is repeatedly performed as much as possible, and more preferably, the washing is performed more than twice.
In order to obtain a high purity silica, distilled water is preferably used, and the temperatures of the acid solution and of the water to be used are preferably 85xcx9c95xc2x0 C., that is, hot water.
In a different embodiment of the present invention, in case of using an acid solution at a temperature of 85xcx9c95xc2x0 C., since the rice bran is washed while being agitated, a device is additionally required, since the temperature of the acid solution is too low to wash at a high speed.
However, in case of washing the rice bran in a boiling acid solution at a temperature of 100xc2x0 C., since the rice bran and the solution are continuously moving while the solution is boiling, organic substances contained in the rice bran can be quickly and effectively removed even without agitation.
(c) Dehydrating Process Under Pressure:
A feature of the present invention is that the rice bran is pressurized by a press so to be dehydrated during a dehydrating process, unlike in the conventional silica preparation method.
Since moisture is removed under a constant pressure, the follow-up process, that is, the drying process, can be performed within a short time period and residual impurities which have not been removed in the washing process can be removed.
In case of dehydrating the rice bran under pressure below 0.005 Mpa, impurities contained in the rice bran is increased, and it is impossible to obtain silica of a purity of more than 99.99%.
Accordingly, the pressure to be applied in the dehydrating is suitably 0.01xcx9c15 Mpa, and more preferably, 0.1xcx9c10 Mpa. Arbitrarily, a washing process may be additionally performed after the dehydrating process under pressure.
In addition, as for the washing process (a), after washing the rice bran, a dehydrating process may be performed to remove impurities.
(d) Drying Process:
A typical drying process can be performed. For example, a hot air drying at about 110xc2x0 C. may be performed, or drying may be performed by using a microwave oven.
The washed and dehydrated rice bran is dried in the microwave oven preferably for 15xcx9c400 seconds, and more preferably, for 30xcx9c300 seconds.
If the drying time period in the microwave oven is less than 15 seconds, the purity of the finally obtained product is low due to a carbon impurity, while if the drying time period in the microwave oven exceeds 400 seconds, the purity of the finally obtained product is also low.
(e) First Burning Process:
The rice bran which have undergone the above processes are agitated and pulverized at a temperature of 300xcx9c500xc2x0 C., and at the same time, burned until the smoke dies off. After undergoing the burning process, the volume of the rice bran is reduced by 1.6xcx9c1.8 times.
During the first burning process, the rice bran becomes fragile, so that they are easily pulverized. If the rice bran is not agitated in this process, the purity of silica becomes low and the content of NO2 is finally increased.
Concurrently performing of the burning and pulverizing is done to increase the burning speed by making the particles of the rice bran small and to completely remove impurities possibly remaining in the rice bran.
In this process, the smoke generated in a heating chamber is extracted by using a pump or a fan to separate soot. Without the additional process in which the smoke generated in the heating chamber is removed by using the pump and so on to separate soot, more of harmful components, for example, CO and NO2, would be contained in the smoke, which is not good for the environment. The separated soot can be utilized to a different use, and in this sense that a by-product detrimental to the environment is utilized, the present invention is very useful.
In addition, the reason why the smoke is removed is to collect the soot, and soot which has not been filtered finally sinks down to the bottom of a water tank filled with water.
The amount of soot sunk down to the bottom of the water tank is about 2.8xcx9c3.2%.
Useful components such as phytine, phytic acid (C6H6[OPO(OH)6] and xylitol (CH2OH(CHOH)3CH2OH) can be separated from the supernatant of the water tank, which can be used as a sterilizing water for seeds for seeding. Table 3 below shows an analysis on the components contained in the soot.
(f) Secondary Burning Process:
The rice bran which have undergone the above processes are burned in a heating device at 700xcx9c1000xc2x0 C. while supplying air or oxygen.
Smoke generated during the secondary burning process is removed by using a fan or a pump, and a soot processing method is performed in the same manner as in the first burning process.
The two-step burning of the rice bran is performed for the following reason. If burning is performed together with agitating and pulverizing in the secondary burning process, since the inner wall of a preferred heating device is made of quartz, the quartz inner wall may be broken due to the an agitator and crusher. Also, after undergoing the first burning process, the volume of the rice bran is reduced by 1.6 times to 1.8 times, so that the necessary fabrication sectional area of the heating device for the secondary burning made of high-priced quartz is reduced to be economical. In addition, as the rice bran undergo the first burning process, their impurities are considerably removed, and thus, the rice bran having a high silica purity undergo the secondary oxide-burning in a separate process, so that a high purity silica can be obtained.
In the present invention, solid state silica or amorphous silica is obtained according to the temperature of the secondary burning. That is, solid silica is obtained at a temperature of not less than 950xc2x0 C., and amorphous silica is obtained at a temperature lower than 950xc2x0 C.
The burning temperature is preferably 700xcx9c1000xc2x0 C., and in case of amorphous silica typically used for semiconductors, the burning temperature is more preferably 700xcx9c800xc2x0 C.
The burning time period in the secondary burning process is preferably 10xcx9c60 minutes, and more preferably, 10xcx9c40 minutes.
Specific reaction conditions and obtained result values of exemplary embodiments of the present invention are shown in Table 1 and Table 3.
The present invention will now be further illustrated by the following examples. It will be apparent to those having ordinary skill in the field that these examples are given only to explain the present invention more clearly, but the invention is not limited to the examples given.